A lot happens in a first impression: we make quick, subconscious assessments about people based purely on their physical appearance and behaviors. And in many social interactions — such as the partners we choose, relationships we make in business and friendship, for example — trust is a critical issue. “If your face looks trustworthy, people [are] more likely to collaborate with you, exchange information, share resources and offer themselves as potential mates,” says the study’s lead author, Karel Kleisner of Charles University in Prague. “Our study shows that perception of trustworthiness may be indirectly affected by eye color via its association with face shape.”

Kleisner and his colleagues took photos of 40 men and 40 women and asked 238 students to rate the images on trustworthiness. Based on previous evidence linking eye color to psychological and social factors such as shyness and extroversion, they ran two rounds of testing — one in which they presented photographs of subjects retaining their natural eye color and another in which the images were retouched so brown-eyed subjects became blue-eyed and vice versa. Each time, the participants were asked to rate how much they would trust the men and women on a 1-to-10 scale, with a score of 1 representing the highest level of trustworthiness.

In the first round of testing, researchers discovered that brown eyes were deemed more trustworthy than blue, regardless of whether they belonged to a man or woman. Eye color was trumped by face shape for men, however. When researchers showed participants pictures of men’s faces that were similar except for their eye color, both men and women saw round-faced men with larger mouths and wider chins as more trustworthy than those with longer faces, regardless of their eye color. The same influence wasn’t seen with women’s face shapes.

Earlier studies on how facial features are perceived suggest that rounder faces with broader chins, wider mouths with upward-pointing corners, bigger eyes and eyebrows that are closer together tend to engender trust. These facial features also happen to be more common among brown-eyed people. Conversely, blue-eyed faces are typically smaller, pointier, longer and with eyebrows situated farther apart, which people tend to interpret as less reliable.

“The longer chin of blue-eyed individuals makes them look less baby-faced. It has been shown that higher ratings for honesty are given to more baby-faced people with shorter chins and lower-positioned facial features,” the authors write.

So why would supposedly shifty features endure in a population? They speculate that the connection could be linked to a form of sexual selection. It’s possible, for example, that blue eyes were more of a novel physical characteristic many years ago. At the time, men may have preferred the newer, more exotic female feature over other facial traits that conveyed trustworthiness and familiarity, contributing to the perpetuation of the suite of facial features associated with lower trustworthiness.

“The faces of brown-eyed people would be seen as more trustworthy because they represent a biosocial adaptation that has been established for millions of years. Nevertheless, the blue-eyed phenotype must have provided its bearers with some kinds of advantage to offset the loss of perceived trustworthiness,” the authors write. And while decisions about how reliable people may be aren’t based on eye color or face shape alone, they may establish a subconscious bias that’s worth recognizing, they say.